Estimating Cobalt by UV-Vis Spectroscopy: The [CoCl?]²? Acetone Method

Central Laboratory, Mitra S. K. Private Limited

Introduction

Cobalt is an essential transition metal with wide industrial applications — from rechargeable lithium-ion batteries to high-strength alloys and catalytic systems.
Accurate determination of cobalt content is crucial for process control, especially in battery recycling, metallurgical refining, and quality testing. Among the many analytical routes, UV-Visible spectrophotometry offers a simple yet powerful tool. When Cobalt (II) interacts with chloride ions in an acetone medium, it forms a deep blue tetrahedral complex, [CoCl₄]²⁻, whose intensity can be directly correlated to cobalt concentration. This method delivers high precision, selectivity, and reproducibility, making it ideal for both high- and low-level cobalt estimation.

Chemical Principle

In aqueous media, Cobalt (II) ions exist as the pink octahedral aqua complex:
[Co(H_2 O)_6 ]^(2+)
Upon the addition of excess chloride ions (Cl⁻), a ligand substitution occurs, forming the blue tetrahedral [CoCl₄]²⁻ complex:
[Co(H_2 O)_6 ]^(2+)+4Cl^-⇌[CoCl_4 ]^(2-)+6H_2 O
The color intensity of this complex is maximized in acetone, where water activity is low and the tetrahedral geometry is stabilized. The resulting solution exhibits a sharp absorption band at 692 nm, characteristic of the d–d transition in [CoCl₄]²⁻. This makes the absorbance at 692 nm a direct measure of cobalt concentration.

Step-by-Step Experimental Procedure

1. Sample Preparation:

• For low-Co samples (<10%): weigh 0.5 g.
• For high-Co samples (10–30%): weigh 0.1 g.
• Digest with perchloric acid (HClO₄) and concentrated HCl.
• Heat gently until full oxidation of Co species.
• Cool, filter, and transfer the solution into a 100 mL volumetric flask; dilute to the mark with distilled water.

2. Calibration Curve:

Prepare a Cobalt Reference Solution:

• Dissolve 0.05 g CRM in 100 mL water → [Co] = 0.288 g/L.
• Prepare appropriate calibration standards series by pipetting CRM aliquots.

3. Sample Measurement:

Sample is measured at 692 nm against a reagent blank.

4. Calculation:

"Co (%)"=(C_measured×V_digest)/(W_sample×10)×100

Where:

= Co concentration from calibration curve (g/L)
= Digest volume (mL, typically 100)
= Sample weight (g)
10= Final dilution volume used for UV-Vis test (mL)

Conclusion

The [CoCl₄]²⁻ acetone method is a robust and elegant approach for cobalt estimation.
Its foundation lies in simple ligand field chemistry, yet it offers the precision expected of advanced spectroscopic methods. By exploiting the deep blue color of the tetrahedral complex, this method combines accuracy, selectivity, and speed, making it ideal for routine laboratory analysis in both industrial and research settings.

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